Identification of an amino acid residue in ATP-binding cassette transport G1 critical for mediating cholesterol efflux

Identification of amino acid residues in the MT-loop of MT1-MMP critical for its ability to cleave low-density lipoprotein receptor

Low-density lipoprotein receptor (LDLR) mediates clearance of plasma LDL cholesterol, preventing the development of atherosclerosis. We previously demonstrated that membrane type 1-matrix metalloproteinase (MT1-MMP) cleaves LDLR and exacerbates the development of atherosclerosis. Here, we investigated determinants in LDLR and MT1-MMP that were critical for MT1-MMP-induced LDLR cleavage. We observed that deletion of various functional domains in LDLR or removal of each of the five predicted cleavage sites of MT1-MMP on LDLR did not affect MT1-MMP-induced cleavage of the receptor. Removal of the hemopexin domain or the C-terminal cytoplasmic tail of MT1-MMP also did not impair its ability to cleave LDLR. On the other hand, mutant MT1-MMP, in which the catalytic domain or the MT-loop was deleted, could not cleave LDLR. Further Ala-scanning analysis revealed an important role for Ile at position 167 of the MT-loop in MT1-MMP’s action on LDLR. Replacement of Ile167 with Ala, Thr, Glu, or Lys resulted in a marked loss of the ability to cleave LDLR, whereas mutation of Ile167 to a non-polar amino acid residue, including Leu, Val, Met, and Phe, had no effect. Therefore, our studies indicate that MT1-MMP does not require a specific cleavage site on LDLR. In contrast, an amino acid residue with a hydrophobic side chain at position 167 in the MT-loop is critical for MT1-MMP-induced LDLR cleavage.

Membrane type 1 matrix metalloproteinase promotes LDL receptor shedding and accelerates the development of atherosclerosis

Plasma low-density lipoprotein (LDL) is primarily cleared by LDL receptor (LDLR). LDLR can be proteolytically cleaved to release its soluble ectodomain (sLDLR) into extracellular milieu. However, the proteinase responsible for LDLR cleavage is unknown. Here we report that membrane type 1-matrix metalloproteinase (MT1-MMP) co-immunoprecipitates and co-localizes with LDLR and promotes LDLR cleavage. Plasma sLDLR and cholesterol levels are reduced while hepatic LDLR is increased in mice lacking hepatic MT1-MMP. Opposite effects are observed when MT1-MMP is overexpressed. MT1-MMP overexpression significantly increases atherosclerotic lesions, while MT1-MMP knockdown significantly reduces cholesteryl ester accumulation in the aortas of apolipoprotein E (apoE) knockout mice. Furthermore, sLDLR is associated with apoB and apoE-containing lipoproteins in mouse and human plasma. Plasma levels of sLDLR are significantly increased in subjects with high plasma LDL cholesterol levels. Thus, we demonstrate that MT1-MMP promotes ectodomain shedding of hepatic LDLR, thereby regulating plasma cholesterol levels and the development of atherosclerosis.

Identification of amino acid residues in the ligand binding repeats of LDL receptor important for PCSK9 binding

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes LDL receptor (LDLR) degradation, increasing plasma levels of LDL cholesterol and the risk of cardiovascular disease. We have previously shown that, in addition to the epidermal growth factor precursor homology repeat-A of LDLR, at least three ligand-binding repeats (LRs) of LDLR are required for PCSK9-promoted LDLR degradation. However, how exactly the LRs contribute to PCSK9’s action on the receptor is not completely understood. Here, we found that substitution of Asp at position 172 in the linker between the LR4 and LR5 of full-length LDLR with Asn (D172N) reduced PCSK9 binding at pH 7.4 (mimic cell surface), but not at pH 6.0 (mimic endosomal environment). On the other hand, mutation of Asp at position 203 in the LR5 of full-length LDLR to Asn (D203N) significantly reduced PCSK9 binding at both pH 7.4 and pH 6.0. D203N also significantly reduced the ability of LDLR to mediate cellular LDL uptake, whereas D172N had no detectable effect. These findings indicate that amino acid residues in the LRs of LDLR play an important role in PCSK9 binding to the receptor.

Control of insulin granule formation and function by the ABC transporters ABCG1 and ABCA1 and by oxysterol binding protein OSBP

In pancreatic β-cells, insulin granule membranes are enriched in cholesterol and are both recycled and newly generated. Cholesterol’s role in supporting granule membrane formation and function is poorly understood. ATP binding cassette transporters ABCG1 and ABCA1 regulate intracellular cholesterol and are important for insulin secretion. RNAi inter­ference–induced depletion in cultured pancreatic β-cells shows that ABCG1 is needed to stabilize newly made insulin granules against lysosomal degradation; ABCA1 is also involved but to a lesser extent. Both transporters are also required for optimum glucose-stimulated insulin secretion, likely via complementary roles. Exogenous cholesterol addition rescues knockdown-induced granule loss (ABCG1) and reduced secretion (both transporters). Another cholesterol transport protein, oxysterol binding protein (OSBP), appears to act proximally as a source of endogenous cholesterol for granule formation. Its knockdown caused similar defective stability of young granules and glucose-stimulated insulin secretion, neither of which were rescued with exogenous cholesterol. Dual knockdowns of OSBP and ABC transporters support their serial function in supplying and concentrating cholesterol for granule formation. OSBP knockdown also decreased proinsulin synthesis consistent with a proximal endoplasmic reticulum defect. Thus, membrane cholesterol distribution contributes to insulin homeostasis at production, packaging, and export levels through the actions of OSBP and ABCs G1 and A1.

ATP-binding cassette transporters and cholesterol translocation

Cholesterol, a major component of mammalian cell membranes, plays important structural and functional roles. However, accumulation of excessive cholesterol is toxic to cells. Aberrant cholesterol trafficking and accumulation is the molecular basis for many diseases, such as atherosclerotic cardiovascular disease and Tangier's disease. Accumulation of excessive cholesterol is also believed to contribute to the early onset of Alzheimer's disease. Thus, cellular cholesterol homeostasis is tightly regulated by uptake, de novo synthesis, and efflux. Any surplus of cholesterol must either be stored in the cytosol in the form of esters or released from the cell. Recently, several ATP-binding cassette (ABC) transporters, such as ABCA1, ABCG1, ABCG5, and ABCG8 have been shown to play important roles in the regulation of cellular cholesterol homeostasis by mediating cholesterol efflux. Mutations in ABC transporters are associated with several human diseases. In this review, we discuss the physiological roles of ABC transporters and the underlying mechanisms by which they mediate cholesterol translocation.

Characterization of the role of a highly conserved sequence in ATP binding cassette transporter G (ABCG) family in ABCG1 stability, oligomerization, and trafficking

ATP-binding cassette transporter G1 (ABCG1) mediates cholesterol and oxysterol efflux onto lipidated lipoproteins and plays an important role in macrophage reverse cholesterol transport. Here, we identified a highly conserved sequence present in the five ABCG transporter family members. The conserved sequence is located between the nucleotide binding domain and the transmembrane domain and contains five amino acid residues from Asn at position 316 to Phe at position 320 in ABCG1 (NPADF). We found that cells expressing mutant ABCG1, in which Asn316, Pro317, Asp319, and Phe320 in the conserved sequence were replaced with Ala simultaneously, showed impaired cholesterol efflux activity compared with wild type ABCG1-expressing cells. A more detailed mutagenesis study revealed that mutation of Asn316 or Phe 320 to Ala significantly reduced cellular cholesterol and 7-ketocholesterol efflux conferred by ABCG1, whereas replacement of Pro317 or Asp319 with Ala had no detectable effect. To confirm the important role of Asn316 and Phe320, we mutated Asn316 to Asp (N316D) and Gln (N316Q), and Phe320 to Ile (F320I) and Tyr (F320Y). The mutant F320Y showed the same phenotype as wild type ABCG1. However, the efflux of cholesterol and 7-ketocholesterol was reduced in cells expressing ABCG1 mutant N316D, N316Q, or F320I compared with wild type ABCG1. Further, mutations N316Q and F320I impaired ABCG1 trafficking while having no marked effect on the stability and oligomerization of ABCG1. The mutant N316Q and F320I could not be transported to the cell surface efficiently. Instead, the mutant proteins were mainly localized intracellularly. Thus, these findings indicate that the two highly conserved amino acid residues, Asn and Phe, play an important role in ABCG1-dependent export of cellular cholesterol, mainly through the regulation of ABCG1 trafficking.

HMG-CoA reductase inhibitors, simvastatin and atorvastatin, downregulate ABCG1-mediated cholesterol efflux in human macrophages

Recent studies showed that statins reduce ATP-binding cassette transporter A1 expression and ATP-binding cassette transporter A1-mediated cholesterol efflux in macrophages, whereas the effect of statins on ATP-binding cassette transporter G1 (ABCG1), another important lipid transporter, is still unclear. Here, we investigated the expression and functionality of ABCG1 on statins in THP-1-derived macrophages and human peripheral blood mononuclear cells. Simvastatin and atorvastatin were used in this study. Treatment with statins significantly decreased ABCG1-mediated cholesterol efflux in human macrophages (from 33.8% ± 2.8% of control to 22.9% ± 1.7% of 10 µM simvastatin or to 23.3% ± 3.3% of 10 µM atorvastatin; P < 0.01, n = 4), whereas the protein expression of ABCG1 remained unaltered on statins. Further analysis revealed that 2 major ABCG1 isoforms responded to statins differently. The expression of ABCG1-S, which exhibited higher activity in cholesterol efflux than ABCG1-L, was significantly decreased on statins compared with increased expression of ABCG1-L. The results suggested that the proportion change of ABCG1 isoform expressions could contribute to reduced ABCG1 functionality under treatment with statins. The effects of statins on ABCG1 isoform expression and functionality were reversed by low-dose liver X receptor agonist, TO-901317, indicating that statins' downregulation of ABCG1 functionality was likely through liver X receptor-dependent pathway. In conclusion, simvastatin and atorvastatin decreased ABCG1-mediated cholesterol efflux in human macrophages without alteration of total ABCG1 protein level. The proportion change of ABCG1 isoforms expressions may be involved in the down-regulation of ABCG1 functionality by statins, which provided a novel mechanism for the regulation of ABCG1 activity.

Characterization of the role of EGF-A of low density lipoprotein receptor in PCSK9 binding

Proprotein convertase subtilisin kexin-like 9 (PCSK9) promotes the degradation of low density lipoprotein receptor (LDLR) and plays an important role in regulating plasma LDL-cholesterol levels. We have shown that the epidermal growth factor precursor homology domain A (EGF-A) of the LDLR is critical for PCSK9 binding at the cell surface (pH 7.4). Here, we further characterized the role of EGF-A in binding of PCSK9 to the LDLR. We found that PCSK9 efficiently bound to the LDLR but not to other LDLR family members. Replacement of EGF-A in the very low density lipoprotein receptor (VLDLR) with EGF-A of the LDLR promoted the degradation of the mutant VLDLR induced by PCSK9. Furthermore, we found that PCSK9 bound to recombinant EGF-A in a pH-dependent manner with stronger binding at pH 6.0. We also identified amino acid residues in EGF-A of the LDLR important for PCSK9 binding. Mutations G293H, D299V, L318D, and L318H reduced PCSK9 binding to the LDLR at neutral pH without effect at pH 6.0, while mutations R329P and E332G reduced PCSK9 binding at both pH values. Thus, our findings reveal that EGF-A of the LDLR is critical for PCSK9 binding at the cell surface (neutral pH) and at the acidic endosomal environment (pH 6.0), but different determinants contribute to efficient PCSK9 binding in different pH environments.

ATP-binding cassette transporters and cholesterol translocation

Cholesterol, a major component of mammalian cell membranes, plays important structural and functional roles. However, accumulation of excessive cholesterol is toxic to cells. Aberrant cholesterol trafficking and accumulation is the molecular basis for many diseases, such as atherosclerotic cardiovascular disease and Tangier's disease. Accumulation of excessive cholesterol is also believed to contribute to the early onset of Alzheimer's disease. Thus, cellular cholesterol homeostasis is tightly regulated by uptake, de novo synthesis, and efflux. Any surplus of cholesterol must either be stored in the cytosol in the form of esters or released from the cell. Recently, several ATP-binding cassette (ABC) transporters, such as ABCA1, ABCG1, ABCG5, and ABCG8 have been shown to play important roles in the regulation of cellular cholesterol homeostasis by mediating cholesterol efflux. Mutations in ABC transporters are associated with several human diseases. In this review, we discuss the physiological roles of ABC transporters and the underlying mechanisms by which they mediate cholesterol translocation. © 2013 IUBMB Life, 2013.

Expanding roles of ABCG1 and sterol transport

PURPOSE OF REVIEW

To offer a comprehensive review on the role of ABCG1 in cellular sterol homeostasis.

RECENT FINDINGS

Early studies with Abcg1 mice indicated that ABCG1 was crucial for tissue lipid homeostasis, especially in the lung. More recent studies have demonstrated that loss of ABCG1 has wide-ranging consequences and impacts lymphocyte and stem cell proliferation, endothelial cell function, macrophage foam cell formation, as well as insulin secretion from pancreatic β cells. Recent studies have also demonstrated that ABCG1 functions as an intracellular lipid transporter, localizes to intracellular vesicles/endosomes, and that the transmembrane domains are sufficient for localization and transport function.

SUMMARY

ABCG1 plays a crucial role in maintaining intracellular sterol and lipid homeostasis. Loss of this transporter has significant, cell-type-specific consequences ranging from effects on cellular proliferation, to surfactant production and/or insulin secretion. Elucidation of the mechanisms by which ABCG1 affects intracellular sterol flux/movement should provide important information that may link ABCG1 to diseases of dysregulated tissue lipid homeostasis.

Characterization of palmitoylation of ATP binding cassette transporter G1: effect on protein trafficking and function

ATP-binding cassette transporter G1 (ABCG1) mediates cholesterol efflux onto lipidated apolipoprotein A-I and HDL and plays a role in various important physiological functions. However, the mechanism by which ABCG1 mediates cholesterol translocation is unclear. Protein palmitoylation regulates many functions of proteins such as ABCA1. Here we investigated if ABCG1 is palmitoylated and the subsequent effects on ABCG1-mediated cholesterol efflux. We demonstrated that ABCG1 is palmitoylated in both human embryonic kidney 293 cells and in mouse macrophage, J774. Five cysteine residues located at positions 26, 150, 311, 390 and 402 in the NH2-terminal cytoplasmic region of ABCG1 were palmitoylated. Removal of palmitoylation at Cys311 by mutating the residue to Ala (C311A) or Ser significantly decreased ABCG1-mediated cholesterol efflux. On the other hand, removal of palmitoylation at sites 26, 150, 390 and 402 had no significant effect. We further demonstrated that mutations of Cys311 affected ABCG1 trafficking from the endoplasmic reticulum. Therefore, our data suggest that palmitoylation plays a critical role in ABCG1-mediated cholesterol efflux through the regulation of trafficking.

Genetics of cholesterol efflux

Plasma levels of high-density lipoprotein cholesterol (HDL-C) show an inverse association with coronary heart disease (CHD). As a biological trait, HDL-C is strongly genetically determined, with a heritability index ranging from 40 % to 60 %. HDL represents an appealing therapeutic target due to its beneficial pleiotropic effects in preventing CHD. This review focuses on the genetic basis of cellular cholesterol efflux, the rate-limiting step in HDL biogenesis. There are several monogenic disorders (e.g., Tangier disease, caused by mutations within ABCA1) affecting HDL biogenesis. Importantly, many disorders of cellular cholesterol homeostasis cause a reduced HDL-C. We integrate information from family studies and linkage analyses with that derived from genome-wide association studies (GWAS) and review the recent identification of micro-RNAs (miRNA) involved in cellular cholesterol metabolism. The identification of genomic pathways related to HDL may help pave the way for novel therapeutic approaches to promote cellular cholesterol efflux as a therapeutic modality to prevent atherosclerosis.